The present invention relates to a differential pressure indicator for use with an oil filter or the like, for electrically detecting when a differential pressure across a filter element exceeds a predetermined level because of a growing mass of particles trapped in the filter element.
Oil filters are provided in oil passages and used to filter, or remove contaminant particles from, oil flowing the through the oil passages thereby purifying the oil. An oil filter has a filter element which is usually made of paper. The filter element removes or separates suspended particulate matter such as dust particles or impurities from oil which flows through the filter element. As the filter element is used over a long period of time, trapping a large amount of contaminant particles, the filter element tends to become clogged by the trapped contaminant. When the filter element is clogged to a certain extent, it has to be replaced with a new one. Usually, the filter element is replaced at intervals of time determined by the regular maintenance schedules of an apparatus such as an engine, a hydraulic pressure device, or the like with which the oil filter is associated. According to such replacement practice, however, the filter element is replaced irrespective of the extent to which it is actually clogged. For example, even if a filter element has not yet been contaminated enough and does not require replacement, it will nevertheless be replaced with a new one when the scheduled time for replacement is reached. Such procedure is undesirable because the filter element, which is still useful, is wasted. Conversely, even if the contamination of a filter element is too great, requiring replacement immediately, no replacement will be carried out unless the scheduled replacement time is reached. This is also not a desirable procedure because the apparatus associated with the oil filter may be adversely affected by the excessive clogging of the filter element.
For proper detection of filter element clogging, it has been proposed to use a differential pressure indicator for detecting a differential pressure which has been developed across a filter element by the clogging of the filter element. When the filter element has been progressively clogged to the extent that the differential pressure exceeds a predetermined level, the differential pressure is electrically detected by the differential pressure indicator. Such detection of the differential pressure then indicates that the filter element should be replaced with a new one.
One problem associated with use of the differential pressure indicator is that since any differential pressure in excess of the predetermined level is automatically detected, the differential pressure indicator is actuated even when such a differential pressure is developed by causes other than the clogging of the filter element. For example, when the oil flowing through the oil filter has a high degree of viscosity at low temperature, the differential pressure across the filter element is increased even if the filter element is not clogged, and the differential pressure indicator detects the increase in the differential pressure and issues an alarm, producing a false indication of the clogging of the filter element.